The present invention relates to an electrophotographic photosensitive member, particularly to one characterized by having a protective layer comprising specific particles and a specific resin, and also to a process cartridge and an electrophotographic apparatus including such a photosensitive member.
An electrophotographic photosensitive member is subjected to a repetition of an image forming cycle including steps of charging, exposure, development, transfer, cleaning, charge removal, etc. An electrostatic latent image formed by the charging and exposure is developed with a fine powdery developer called a toner to form a toner image on the photosensitive member. The toner image is then transferred onto a transfer(-receiving) material, such as paper, but all the toner is not transferred but a portion thereof remains as a residual toner on the photosensitive member.
A large amount of the residual toner, if caused, can promote a further transfer failure to result in a toner image on the transfer material with noticeable lack of portion of image and image uniformity. Further, the residual toner causes problems, such as melt-sticking and filming of the toner onto the photosensitive member. In order to cope with these problems, an electrophotographic photosensitive member is required to have a surface layer with improved releasability.
Further, an electrophotographic photosensitive member is subjected to direct application of electrical and mechanical external forces, so that the photosensitive member is required to be durable against such forces. More specifically, the photosensitive member is required to be durable against the occurrences of surface abrasion and scars due to rubbing and surface layer degradation due to attachment of active substances, such as ozone and NOx occurring during the charging of the photosensitive member.
In order to comply with the above-mentioned requirements of the photosensitive member, it has been proposed to dispose various protective layers. For example, Japanese Laid-Open Patent Application (JP-A) 57-30846 discloses a protective layer comprising a resin to which a metal oxide is added as electroconductive power for resistivity control.
The dispersion of electroconductive powder in such a protective layer of an electrophotographic photosensitive member is performed principally for the purpose of controlling the electrical resistivity of the protective layer per se to prevent an increase in residual potential in the photosensitive member liable to be caused along with the repetition of the electrophotographic image forming cycles. It is known than an appropriate range of volume resistivity of a protective layer is 1010 to 1015 ohm.cm. The resistivity in the above-mentioned range of protective layer is liable to be affected by ionic conduction and is therefor liable to result in a remarkable change in resistivity due to an environmental change. Particularly, in the case of a resinous film containing metal oxide powder dispersed therein, it has been very difficult to keep the resistivity of the protective layer in the above-mentioned range under various environmental conditions since the metal oxide powder surface exhibits a high moisture absorptivity. Further, many resins per se exhibit high moisture absorptivity and are liable to lower the resistivity of the protective layer formed therefrom.
Particularly, in a high-humidity environment, the surface layer of a photosensitive member is liable to have a lower resistivity by standing or repetitive surface-attachment of active substances, such as ozone and NOX, and also cause a lowering in toner releasability, thus causing image defects such as image flow and insufficient image uniformity.
In the case of dispersing electroconductive particles in a protective layer, it is generally preferred that the particles have a particle size (diameter) smaller than the wavelength of light incident thereto, that is, at most 0.3 xcexcm, in order to prevent the scattering of incident light due to the dispersed particles. Moreover, electroconductive particles generally tend to agglomerate with each other when dispersed in a resin solution, are difficult to disperse, and even if once dispersed, are liable to cause secondary agglomeration or precipitation, so that it has been difficult to form a resinous film in which fine particles of at most 0.3 xcexcm in particle size are uniformly dispersed. Further, in other to provide a protective layer with a better transparency and a better uniformity of electroconductivity, it is particularly preferred to disperse fine particles (of at most 0.1 xcexcm in primary particle size), but such fine particles are liable to exhibit even worse dispersibility and dispersion stability.
In order to alleviate the above-mentioned difficulties, JP-A 1-306857 has disclosed a protective layer containing a fluorine-containing silane coupling agent or titanate coupling agent, or a compound such as C7F15NCO; JP-A 62-295066 has disclosed a protective layer containing metal or metal oxide fine power subjected to a water-repelling treatment for improved dispersibility and moisture resistance dispersed in a binder resin; and JP-A 2- 50167 has disclosed a protective layer containing metal oxide fine power surface-treated with a titanate coupling agent, a fluorine-containing silane coupling agent or acetoalkoxy-aluminum diisopropylate dispersed in a binder resin.
However, even such a protective layer still shows a lower resistivity to cause image blurring in a high-humidity environment and exhibits insufficient durability against abrasion or scars due to rubbing, thus being not fully satisfactory as a protective layer for providing electrophotographic performances complying with demands for high image qualities in recent years.
On the other hand, the use of fluorinated carbon as moderately electroconductive particles together with various binder resins including a thermosetting phenolic resin for providing a protective layer has been proposed in JP-A 62-19254. However, the resultant protective layer is not sufficient with respect to dispersion of the fluorinated carbon and environmental stability of the resistivity, thus being liable to result in increases in resistivity and residual potential in a low humidity environment, and a lower humidity to cause image blurring in a high humidity environment.
The use of various thermosetting resins, inclusive of a phenolic resin, together with various filler materials, inclusive of a metal oxide, for providing a protective layer, has been proposed in JP-A 5-181299. However, the metal oxide fine particles disclosed therein are non-conductive reinforcing particles preferably having a particle size of 0.05-3 xcexcm. Accordingly, the metal oxide particles are not effective for providing a protective layer exhibiting a low resistivity, and a sufficient consideration has not been paid to the provision of a transparent protective layer.
As described above, it has been very difficult to realize a protective layer satisfying various properties required thereof at a high level.
Accordingly, a generic object of the present invention is to provide an electrophotographic photosensitive member having solved the above-mentioned problems of the conventional electrophotographic photosensitive members.
A more specific object of the present invention is to provide an electrophotographic photosensitive member which is substantially free from an increase in residual potential in a low-humidity environment and is capable of providing high-quality images free from image blurring or image flow in a high-humidity environment.
Another object of the present invention is to provide an electrophotographic photosensitive member which has a surface layer exhibiting excellent releasability and excellent durability against abrasion and scars and thus can maintain high-quality images.
A further object of the present invention is to provide a process cartridge and an electrophotographic apparatus including such an electrophotographic photosensitive member.
According to the present invention, there is provided an electrophotographic photosensitive member, comprising: a support, a photosensitive layer and a protective layer in this order; wherein said protective layer has a thickness of 1-7 xcexcm and comprises a cured phenolic resin and metal particles or metallic oxide particles dispersed therein.
According to the present invention, there is further provided a process cartridge, comprising: the above-mentioned electrophotographic photosensitive member and at least one means selected form the group consisting of charging means, developing means and cleaning means; said electrophotographic photosensitive member and said at least one means being integrally supported and detachably mountable to a main assembly of an electrophotographic apparatus.
The present invention further provides an electrophotographic apparatus, comprising: the abovementioned electrophotographic photosensitive member, and charging means, developing means and transfer means respectively disposed opposite to the electrophotographic photosensitive member.